1. Field of the invention
The present invention relates, in general, to image forming apparatus. More specifically, the invention relates to an electrostatic copying machine which forms a visible image corresponding to an original document on a paper.
2. Description of the prior art
Generally, in conventional copying machines, a copy paper may be offset in the direction perpendicular to the feeding direction of a copy paper, as it is fed from a paper-cassette to a photosensitive drum. This offset results from the manual setting of the paper-cassette by an operator. Since the offset is limited to 3 nm in either direction from the optimum, it has been neglected in conventional copying machines. However, the shifting of the paper causes an offset of a visible image of an original document formed on a copy paper.
Furthermore, since qualities of copy paper vary widely, such as e.g., sheet thickness, smoothness, etc., the actual feeding time interval for feeding a copy paper from a pair of aligning rollers to a photosensitive drum may vary from a prescribed feeding time interval. This is caused by slipping between the copy paper and the aligning roller pair. The result is that it is difficult to form the visible image of an original document at an appropriate position on the copy paper.
It is, therefore, desirable to detect the above-described offset and feeding time interval variations of the copy paper and to adjust the position of the visible image formed on the copy paper. Furthermore, it is convenient to achieve such adjustment even though the size of a copy paper is changed.